Tamper-proof electronic coatings

ABSTRACT

Disclosed is a method of forming tamper-proof coatings on electronic devices. The method comprises applying a coating of a silica precursor resin and an inorganic salt onto the electronic device, wherein the inorganic salt is one which reacts with a wet etch to yield an acid or base that damages the electronic device. The coated electronic device is then heated at a temperature sufficient to convert the silica precursor resin to a silica containing ceramic matrix.

This application is a division of application Ser. No. 08/548,851 filedOct. 26, 1995 which application is now: U.S. Pat. No. 5,693,701.

BACKGROUND OF THE INVENTION

The dissection of electronic devices is a major source of informationfor both commercial competitors as well as foreign governments. In mostinstances, the devices are analyzed by techniques such ascross-sectioning and etching. The present invention relates to coatingscomprising silica-containing matrices containing inorganic salts whichinhibit this type of examination.

The use of silica-containing ceramic coatings on substrates such aselectronic devices is known in the art. For instance, Haluska et al. inU.S. Pat. Nos. 4,749,631 and 4,756,977 disclose processes for formingsilica coatings on electronic substrates wherein solutions of silicaprecursor resins are applied to substrates followed by heating thecoated substrates in air at a temperature in the range of 200°-1000° C.These references, however, do not describe the use of inorganic saltswithin the coating.

Similarly, the use of fillers within ceramic coatings is also known inthe art. For instance, U.S. Pat. No. 3,986,997 describes a compositioncomprising an acidic dispersion of colloidal silica and hydroxylatedsilsesquioxane in an alcohol-water medium which can be used to applytransparent abrasion resistant coatings on a variety of substrates. Thereference, however, does not describe the use of the inorganic saltsdescribed herein nor the application of a coating on an electronicsubstrate.

U.S. patent application 08/103,142 filed Aug. 09,1993, U.S. Pat. No.5,458,912 teaches tamper-proof coatings on electronic devices. Thecoatings therein comprise silica-containing matrices with materials suchas metals therein which react in oxidizing atmospheres to destroy theunderlying device. This reference, too, does not describe the use of theinorganic salts taught herein.

The present inventors have now discovered that tamper-proof coatings forelectronic circuits can be formed from compositions comprising silicaprecursor resins and inorganic salts.

SUMMARY OF THE INVENTION

The present invention relates to a method of forming a tamper-proofcoating on an electronic substrate and the substrate coated thereby. Themethod comprises first applying a composition comprising a silicaprecursor resin and an inorganic salt onto the substrate. The inorganicsalt used is one which reacts with a wet etch to yield an acid or basethat damages the electronic device. The coated substrate is then heatedat a temperature sufficient to convert the coating composition to aceramic coating.

The present invention also relates to the coating composition comprisingthe above silica precursor resin and an inorganic salt.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the discovery that tamper-proofcoatings can be formed from compositions comprising silica precursorresins and inorganic salts. When these coatings are analyzed by wetetching, the inorganic salts react to yield acids or bases (localizedchanges in pH) which cause uncontrollable destruction of the underlyingmetallization or substrate and, thus, inhibit further examination and/orreverse engineering.

As used in the present invention, the expression "silica containingmatrix" is used to describe the hard coating obtained after heating thesilica precursor resin. This coating contains both amorphous silica(SiO₂) materials as well as amorphous silica-like materials that are notfully free of residual carbon, silanol (Si-OH) and/or hydrogen (whichare obtained upon heating the silica precursor resin) and the inorganicsalts. The expression "filler" is used to describe a finely dividedsolid phase which is distributed within the resin and the final ceramiccoating. The expression "electronic substrate" is meant to include, butis not limited to, electronic devices or electronic circuits such assilicon based devices such as integrated circuits, gallium arsenidebased devices, focal plane arrays, opto-electronic devices, photovoltaiccells and optical devices. The "wet etch" described herein involves theuse of water, alkaline or acidic solutions to etch a variety ofcoatings. This process is well known and commonly used in thesemiconductor industry.

In the process of the present invention a ceramic coating is formed on asubstrate by a process which comprises applying a coating compositioncomprising a silica precursor resin and the inorganic salt onto thesubstrate and then heating the coated substrate at a temperaturesufficient to convert the composition to a silica-containing ceramicmatrix having the inorganic salt distributed therein.

The silica precursor resins which may be used in the invention include,but are not limited to, hydrogen silsesquioxane resin (H-resin),hydrolyzed or partially hydrolyzed R_(n) Si(OR)_(4-n), or combinationsof the above, in which R is an aliphatic, alicyclic or aromaticsubstituent of 1-20 carbon atoms such as an alkyl (eg. methyl, ethyl,propyl), alkenyl (eg. vinyl or allyl), alkynyl (eg. ethynyl),cyclopentyl, cyclohexyl, phenyl etc., and n is 0-3.

The hydrogen silsesquioxane resins (H-resin) which may be used in thisinvention include hydridosiloxane resins of the formula HSi(OH)_(x)(OR)_(y) O_(z/2), in which each R is independently an organic group or asubstituted organic group which, when bonded to silicon through theoxygen atom, forms a hydrolyzable substituent, x=0-2, y=0-2, z=1-3,x+y+z=3. Examples of R include alkyls such as methyl, ethyl, propyl,butyl, etc., aryls such as phenyl, and alkenyls such as allyl or vinyl.As such, these resins may be fully condensed (HSiO_(3/2))_(n) or theymay be only partially hydrolyzed (i.e., containing some Si-OR) and/orpartially condensed (i.e., containing some Si-OH). Although notrepresented by this structure, these resins may contain a small number(eg., less than about 10%) of silicon atoms which have either 0 or 2hydrogen atoms attached thereto due to various factors involved in theirformation or handling.

The above H-resins and methods for their production are known in theart. For example, Collins et al. in U.S. Pat. No. 3,615,272, which isincorporated herein by reference, teach the production of a nearly fullycondensed H-resin which may contain up to 100-300 ppm silanol) by aprocess comprising hydrolyzing trichlorosilane in a benzenesulfonic acidhydrate hydrolysis medium and then washing the resultant resin withwater or aqueous sulfuric acid. Similarly, Bank et al. in U.S. Pat. No.5,010,159, which is hereby incorporated by reference, teach analternative method comprising hydrolyzing hydridosilanes in anarylsulfonic acid hydrate hydrolysis medium to form a resin which isthen contacted with a neutralizing agent.

Other hydridosiloxane resins, such as those described by Frye et al. inU.S. Pat. No. 4,999,397, hereby incorporated by reference, thoseproduced by hydrolyzing an alkoxy or acyloxy silane in an acidic,alcoholic hydrolysis medium, those described in Kokai Patent Nos.59-178749, 60-86017 and 63-107122, or any other equivalenthydridosiloxane, will also function herein.

The second type of silica precursor resin useful herein includeshydrolyzed or partially hydrolyzed compounds of the formula R_(n)Si(OR)_(4-n) in which R and n are as defined above. Some of thesematerials are commercially available, for example, under the tradenameACCUGLASS. Specific compounds of this type includemethyltriethoxysilane, phenyltriethoxysilane, diethyldiethoxysilane,methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane,vinyltrimethoxysilane, tetramethoxysilane, tetraethoxysilane,tetrapropoxysilane, and tetrabutoxysilane. After hydrolysis or partialhydrolysis of these compounds, the silicon atoms therein may be bondedto C, OH or OR groups, but a substantial portion of the material isbelieved to be condensed in the form of soluble Si-O-Si resins.Compounds in which x=2 are generally not used alone as volatile cyclicstructures are generated during pyrolysis, but small amounts of saidcompounds may be cohydrolyzed with other silanes to prepare usefulpreceramic materials.

The coating composition may also contain other ceramic oxide precursors.Examples of such ceramic oxide precursors include compounds of variousmetals such as aluminum, titanium, zirconium, tantalum, niobium and/orvanadium as well as various non-metallic compounds such as those ofboron or phosphorous which may be dissolved in solution, hydrolyzed, andsubsequently pyrolyzed, at relatively low temperatures and relativelyrapid reaction rates to form ceramic oxide coatings.

The above ceramic oxide precursor compounds generally have one or morehydrolyzable groups bonded to the above metal or non-metal, depending onthe valence of the metal. The number of hydrolyzable groups to beincluded in these compounds is not critical as long as the compound issoluble in the solvent. Likewise, selection of the exact hydrolyzablesubstituent is not critical since the substituents are either hydrolyzedor pyrolyzed out of the system. Typical hydrolyzable groups include, butare not limited to, alkoxy, such as methoxy, propoxy, butoxy and hexoxy,acyloxy, such as acetoxy, or other organic groups bonded to said metalor non-metal through an oxygen such as acetylacetonate. Specificcompounds, therefore, include zirconium tetracetylacetonate, titaniumdibutoxy diacetylacetonate, aluminum triacetylacetonate andtetraisobutoxy titanium.

When hydrogen silsesquioxane resin is to be combined with one of theabove ceramic oxide precursors, generally it is used in an amount suchthat the final ceramic coating contains 0.1 to about 30 percent byweight modifying ceramic oxide.

The coating composition may also contain a platinum, rhodium or coppercatalyst to increase the rate and extent of conversion to silica.Generally, any platinum, rhodium or copper compound or complex which canbe solubilized will be functional. For instance, a composition such asplatinum acetylacetonate, rhodium catalyst RhCl₃ S(CH₂ CH₂ CH₂ CH₃)₂ !₃,obtained from Dow Corning Corporation, Midland, Mich., or cupricnaphthenate are all within the scope of this invention. These catalystsare generally added in an amount of between about 5 to 1000 ppmplatinum, rhodium or copper based on the weight of hydrogensilsesquioxane resin.

The inorganic salts used herein are those which react with wet etch toyield acids or bases that cause uncontrolled damage to the underlyingelectronic devices. These include, for example, the sodium, potassium,calcium and ammonium salts of phosphates, halides, nitrates, nitrites,chlorates, sulfates, sulfites, chromates and the like. Examples includesodium fluoride, calcium fluoride, sodium nitrate, sodium sulfate,ammonium fluoride and the like.

The particle size and shape of the above inorganic salts can vary over awide range depending on factors such as the type of salt, the desiredcoating thickness, etc. Powder morphologies include, but are not limitedto powders, particles, flakes and the like.

The amount of inorganic salt used in the present invention can also bevaried over a wide range depending, for example, on the desired qualityof the final coating. Generally, however, the inorganic salts are usedin an amount less than about 90 weight percent of the coating to insurethat enough resin is present to bind the inorganic salt. Obviously,smaller amounts of inorganic salts (eg., 1-5 wt%) can also be used. Apreferred amount of inorganic salt is in the range of about 5 to about80 wt. percent of the coating.

If desired, other materials may also be present in the coatingcomposition. For instance, it is within the scope of the presentinvention to use a material which modifies the surface of the inorganicsalt for better adhesion. Such materials can include, for example,silanes such as glycidoxypropyltrimethoxysilane,mercaptopropyltrimethoxysilane, and vinyltriacetoxysilane. Similarly, itis within the scope of the invention to include suspending agents tosuspend the inorganic salts in the preceramic coating composition. Theseand other optional components are known to those skilled in the art.

According to the process of the invention, the silica precursor resin,inorganic salt and any optional components are applied to the surface ofan electronic device. The surface of the electronic device can be bare(i.e., no passivation) or the circuit can have a primary passivation.Such primary passivation can be, for example, ceramic coatings such assilica, silicon nitride, silicon carbide, silicon oxynitride, siliconoxycarbide, etc. deposited by, for example, CVD such as thermal CVD,PECVD, etc., PVD, or sol-gel approaches. Such primary passivation isknown to those skilled in the art.

The coating according to the present invention can be applied in anymanner, but a preferred method involves dissolving the silica precursorresin in a solvent and dispersing the inorganic salt and any optionalcomponents therein. This dispersion is then applied to the surface ofthe substrate. Various facilitating measures such as stirring and/orheating may be used to dissolve or disperse the silica precursor resinand inorganic salt and create a more uniform application material.Solvents which may be used include any agent or mixture of agents whichwill dissolve or disperse the silica precursor resin and inorganic saltto form a homogenous liquid mixture without affecting the resultantcoating. These solvents can include, for example, alcohols such as ethylor isopropyl, aromatic hydrocarbons such as benzene or toluene, alkanessuch as n-heptane or dodecane, ketones, esters, glycol ethers, or cyclicdimethylpolysiloxanes, in an amount sufficient to dissolve/disperse theabove materials to the concentration desired for application. Generally,enough of the above solvent is used to form a 0.1-80 weight percentsolids mixture, preferably 1-50 wt. percent solids.

If a liquid method is used, the liquid mixture comprising the silicaprecursor resin, inorganic salt, solvent, and, any optional componentsis then coated onto the substrate. The method of coating can be, but isnot limited to, spin coating, dip coating, spray coating or flowcoating. Other equivalent means, however, are also deemed to be withinthe scope of this invention. If desired, the liquid mixture can beselectively deposited as described in U.S. Pat. No. 5,399,441 which isincorporated herein by reference.

The solvent is then allowed to evaporate from the coated substrateresulting in the deposition of the silica precursor resin and inorganicsalt coating. Any suitable means of evaporation may be used such assimple air drying by exposure to an ambient environment, by theapplication of a vacuum or mild heat or during the early stages of theheat treatment. It is to be noted that when spin coating is used, theadditional drying period is minimized as the spinning drives off thesolvent.

Although the above described methods primarily focus on using a solutionapproach, one skilled in the art would recognize that other equivalentmeans (eg., melt coating) would also function herein and arecontemplated to be within the scope of this invention.

The silica precursor resin and inorganic salt coating is then typicallyconverted to a silica-containing ceramic matrix having the inorganicsalt distributed therein by heating it to a sufficient temperature.Generally, the temperature is in the range of about 50° to about 1000°C. depending on the pyrolysis atmosphere. Preferred temperatures are inthe range of about 50° to about 800° C. and more preferably 50°-450° C.to prevent damage to the electronic device. Heating is generallyconducted for a time sufficient to ceramify, generally up to about 6hours, with less than about 3 hours being preferred.

The above heating may be conducted at any effective atmospheric pressurefrom vacuum to superatmospheric and under any effective oxidizing ornon-oxidizing gaseous environment such as those comprising air, ₀₂,ozone, an inert gas (N₂, Ar etc.), ammonia, amines, N₂ O, H₂ etc.

Any method of heating such as the use of a convection oven, rapidthermal processing, hot plate, or radiant or microwave energy isgenerally functional herein. The rate of heating, moreover, is also notcritical, but it is most practical and preferred to heat as rapidly aspossible.

By the above methods a ceramic coating is produced on the substrate. Thethickness of the coating can vary over a wide range (eg., up to 500microns) as described above. These coatings smooth the irregularsurfaces of various substrates, they are relatively defect free and theyhave excellent adhesive properties. As such, they are particularlyuseful for a variety of electronic applications such as protectivelayers. In addition, the coatings are tamper-proof such that examinationby wet etching causes the inorganic salt to be converted into an acid orbase which results in destruction of the electronic device.

Additional coatings may be applied over these coatings if desired. Thesecan include, for example, SiO₂ coatings, SiO₂ / ceramic oxide layers,silicon containing coatings, silicon carbon containing coatings, siliconnitrogen containing coatings, silicon oxygen nitrogen coatings, siliconnitrogen carbon containing coatings and/or diamond like carbon coatings.Methods for the application of such coatings are known in the art andmany are described in U.S. Pat. No.4,756,977, which is incorporatedherein by reference. An especially preferred coating is silicon carbideapplied by the chemical vapor deposition of an organosilicon precursor.One example of such a process is described in U.S. Pat. No. 5,011,706which is incorporated herein by reference. A second example involves thechemical vapor deposition utilizing trimethyl silane as the source gas.The most preferred coating comprises silicon carbide deposited in anon-uniform thickness such that uniform etching is difficult.

The following non-limiting example is included so that one skilled inthe art may more readily understand the invention.

EXAMPLE 1

A coating composition was formed by mixing 3 g CaF₂, 1 g of Hydrogensilsesquioxane resin made by the method of Collins et al. in U.S. Pat.No. 3,615,273, and 0.4 g glycidoxypropyltrimethoxysilane and 2.5 gcyclic polydimethylsiloxane. The above mixture was subjected to a sonicprobe 3 times for 10 seconds each. The coating composition was appliedto the surface of an 11.4 cm square aluminum panel using a 50 cmdrawdown bar. The coating was allowed to dry for 3 hours at roomtemperature. The coated panels were then heated at 400° C. for 1 hour inair.

The resultant coating was 35 micrometers thick. Examination of thecoating under a microscope showed no cracks at 1000×.

What is claimed is:
 1. A method of forming a tamper-proof coating on anelectronic device comprising:applying a coating composition comprising asilica precursor resin and an inorganic salt onto an electronic device,wherein the inorganic salt is one which reacts with a wet etch to yieldan acid or base that damages the electronic device; and heating thecoated substrate at a temperature sufficient to convert the coatingcomposition into a ceramic coating.
 2. The method of claim 1 wherein thecoating composition is applied to the substrate by a process whichcomprises coating the substrate with a liquid mixture comprising asolvent, the silica precursor resin and the inorganic salt and thenevaporating the solvent.
 3. The method of claim 1 wherein the silicaprecursor resin is selected from the group consisting of hydrogensilsesquioxane resin and hydrolyzed or partially hydrolyzed R_(n)Si(OR)_(4-n), wherein R is an aliphatic, alicyclic or aromaticsubstituent of 1-20 carbon atoms and n is 0-3.
 4. The method of claim 1wherein the coated substrate is heated at a temperature in the range ofbetween about 50° C. and about 450° C. for less than about 3 hours. 5.The method of claim 1 wherein the coating composition also containsmodifying ceramic oxide precursors comprising a compound containing anelement selected from the group consisting of titanium, zirconium,aluminum, tantalum, vanadium, niobium, boron and phosphorous wherein thecompound contains at least one hydrolyzable substituent selected fromthe group consisting of alkoxy and acyloxy and the compound is presentin an amount such that the ceramic coating contains 0.1 to 30 percent byweight modifying ceramic oxide.
 6. The method of claim 3 wherein thesilica precursor resin is hydrogen silsesquioxane resin and the coatingcomposition also contains a platinum, rhodium or copper catalyst in anamount of between about 5 and about 500 ppm platinum, rhodium or copperbased on the weight of hydrogen silsesquioxane resin.
 7. The method ofclaim 1 wherein the coating composition also contains a material whichmodifies the surface of the inorganic salt for better adhesion.
 8. Themethod of claim 2 wherein the coating composition also contains an agentwhich assists in suspending the inorganic salt in the solution.
 9. Themethod of claim 1 wherein the inorganic salt is in a form selected fromthe group consisting of powders, particles, and flakes.
 10. The methodof claim 1 wherein the inorganic salt is selected from the groupconsisting of salts of phosphates, halides, nitrates, nitrites,chlorates, sulfates, sulfites and chromates.
 11. The method of claim 1wherein the inorganic salt is present in the coating composition in anamount in the range of about 10 to about 80 weight percent.
 12. Theelectronic device coated by the method of claim 1.